Intestinal uptake of IgG in suckling rats. Distinction between jejunal and ileal epithelial cells demonstrated by simultaneous ultrastructural localization of IgG and acid phosphatase.

During the suckling period, the proximal small intestine of neonatal rats absorbs maternal milk immunoglobulin G (IgG) into the circulation by a receptor-mediated mechanism. At the same time, milk IgG enters vacuoles in the distal small intestine unaided by a receptor and is degraded. To help define the distinction between the handling of IgG by the proximal and distal neonatal small intestine, we simultaneously localized IgG (by peroxidase-labeled antibodies) and acid phosphatase (by a lead phosphate technique) at the ultrastructural level. In the proximal small intestinal epithelial cells, IgG-containing vesicles were separate from acid phosphatase-containing structures, whereas in the distal intestinal epithelial cells, IgG was present together with acid phosphatase in large supranuclear vacuoles. These observations are consistent with the hypothesis that IgG avoids degradation in the proximal intestinal cells because vesicles in which it is transported do not fuse with lysosomes, whereas IgG in the distal small intestine is degraded in lysosomal enzyme-containing organelles.